1. Field of the Invention
The present invention relates to a magnetic disk device, a servo-information writing device, a magnetic disk, and a method of manufacturing a magnetic disk device.
2. Description of the Related Art
In a conventional magnetic disk (see FIG. 8), servo information (information for controlling the position and the speed of a head) required by the magnetic disk device is written at the same frequency (low peak density) across the entire radial width (ranging from the inner circumference to the outer circumference of the magnetic disk), as a seek operation of the magnetic disk necessitates reading the servo information at a fixed sampling frequency. Over the years, technologies for improving the format volume of the conventional magnetic disk and for accurately reading the servo information written onto the conventional magnetic disk have also been disclosed.
For example, Japanese Patent Application Laid-Open No. H5-205419 discloses a technology for improving the format volume of the magnetic disk by reducing the ratio of the servo information occupying the track volume by varying the write frequency at which the servo information is written according to the radial position of the magnetic disk. That is, this technology involves writing the servo information by increasing a bit density towards the outer circumference.
FIG. 9 is schematic for explaining the conventional technology described above. As shown in FIG. 9, a frequency-switching track demarcates an area towards the inner circumference where the servo information is written at a low frequency, and an area towards the outer circumference where the servo information is written at a high frequency. More format volume is obtained by this method of writing servo information than when the servo information is written at the same frequency.
Japanese Patent Application Laid-Open No. 2003-323772 discloses a technology for reducing the time required for head change for every magnetic disk necessitated by eccentricity that occurs due to the magnetic disk rotation center when servo information is written onto it and the magnetic disk rotation center not coinciding when a plurality of magnetic disks with servo information written thereon are incorporated into the magnetic disk device. This technology works by creating concentric tracks on the magnetic disk in its incorporated state, and causing the head to follow the track.
FIG. 10 is a schematic for explaining the conventional technology described above. As shown in FIG. 10, when the magnetic disk rotation center and the magnetic disk rotation center when the servo information is written onto it do not coincide, concentric tracks (one of which is represented by the dashed line A) are created in its incorporated state, and the head is made to follow the track. Thus, an improvement is brought about in terms of being able to easily read the servo information written onto the magnetic disk and reducing the time required for the head change for every magnetic disk.
Another technology involves causing the head to follow the servo information itself. In this case, when a magnetic disk having servo information written at different frequencies in the area towards the inner circumference and the area towards the outer circumference, such as the one disclosed in Japanese Patent Application Laid-Open No. H5-205419, is incorporated in a magnetic disk device, and the magnetic disk rotation center and the magnetic disk rotation center when the servo information is written onto it do not coincide, the head is made to follow the servo information.
FIG. 11 is a schematic for explaining the technology described above. As shown in FIG. 11, three tracks will be affected by the switching of frequency due to an eccentricity magnitude, the three tracks being the frequency-switching track (represented by the dashed line B) and the track on either side of the frequency-switching track. Thus, the format volume of the magnetic disk can be increased by limiting the number of unusable tracks.
However, no improvement can be seen in the format volume by incorporating a magnetic disk with servo information written at different frequencies in the area towards the inner circumference and the area towards the outer circumference (see Japanese Patent Application Laid-Open No. H5-205419) into a magnetic disk device, and creating concentric tracks when the center of the magnetic disk and the center of the magnetic disk when the servo information is written onto it do not coincide, and causing the head to follow the servo information (see Japanese Patent Application Laid-Open No. 2003-323772).
FIG. 12 is a schematic for explaining why the conventional technology described above produces no improvement in the format volume. As shown in FIG. 12, as the tracks that are read are switched in such a way that they are not affected by the eccentricity of the servo information, the tracks that fall in the range of the eccentricity magnitude cannot be used as normal tracks as servo information written at different frequencies are read from these tracks in addition to the servo information from the frequency-switching track (represented by the dashed line C). As a result, there is a loss of format volume.
FIG. 13 is a schematic for explaining the above description in further detail. If the head is made to follow the servo information, according to how the servo information is written, sometimes the servo information in the area in the outer circumference and sometimes the servo information in the area in the inner circumference is read. As a result, the tracks in that range cannot be used as normal tracks, leading to a loss of format volume.